Heat the chicken broth in a medium saucepot over high heat, seasoning it generously with salt. Add the chicken thighs to the pan and bring the broth to a simmer. Cover, and reduce the heat to low. Simmer gently until the chicken is tender, about 40 minutes. Remove the thighs from the liquid, pat them dry, and set them aside on a platter; cover it with foil to keep them warm.

While the chicken is cooking, pour the grapeseed oil into a large pot with high sides, and bring it to 400F over high heat. Set a wire rack in a rimmed baking sheet or over several layers of paper towels (for draining chicken). In a shallow dish, combine the flour with the paprika, celery salt, black pepper, 1 teaspoon salt, and the cayenne. Use a whisk to thoroughly combine the ingredients.

Combine the warm chicken thighs with the buttermilk in a large bowl, coating them completely. Dredge the thighs in the seasoned flour. Then dip the thighs in the buttermilk and dredge them in the seasoned flour once more, to double-coat the chicken. Shake off any excess flour.

Fry the chicken, 2 pieces at a time, in the hot oil until deep golden brown, 30 seconds to 1 minute. Drain on the wire rack and serve immediately.